With the alpha release of Laravel Spark, it includes an installer for scaffolding out everything for a new Laravel app but it left out the ability to add it to an existing app. Mike Bronner created a new package to fill this void:
This is very much a pre-release version. As long as Laravel Spark is not stable, neither is this package. Please test on a new branch of your Laravel app, so in case it doesn’t work as expected you won’t lose anything. If it does work, just PR the branch into your release branch for your app once you are satisfied.
You can find the package on Github with instructions.